In a particular application of this invention, an electrode furnace (EF) enables rapid heating of a sample material used to create gases. These gases are then analyzed for their composition using a variety of scientific methods. The EF operates by generating a high current which is passed through a conductive crucible. Current is conducted through the crucible using electrode contacts. The current heats the crucible and any sample material therein. As used herein, the term electrode defines an electromechanical connection between a conductive material and a load.
Prior art systems have used large mains-frequency (50 Hz-60 Hz) power supplies to generate the high currents necessary to rapidly produce enough heat to drive off gases in the sample material. These types of linear power supplies require a large iron core transformer making them bulky and difficult to integrate into the EF. Although higher frequency switching supplies can be used for reducing the transformer size, these types of switching supplies often have problems when delivering a high current to the load. This is primarily due to the stray inductance created by the flexible lead wire used to connect the transformer with the electrode, the electrode inductance, and the transformer leakage inductance. The stray inductance results in an impedance that increases with frequency and is in series with the crucible resistance. At normal mains input frequencies of 50 Hz-60 Hz, the stray inductance contributes an insignificant amount of inductive reactance to the system. Therefore, the transformer secondary circuit impedance is dominated by the crucible resistance at 50 Hz or 60 Hz. At frequencies normally utilized by switching power supplies, the inductive reactance created by the stray inductance can be many times that of the crucible resistance.
FIG. 1 is a block diagram illustrating a prior art EF system 100 using a phase chopper supply. As described herein, the EF system 100 is used for heating a crucible 109. A mains input voltage 101 is supplied to a conduction angle or phase controlled chopper 103 used to regulate the output current of a step down transformer 105. The chopper limits the input waveform to the transformer to less than one full cycle by use of an SCR or similar device. The transformer 105 works to supply a substantially high current through a flexible connection 107 to a crucible 109 used for holding analytical samples. The flexible connection 107 consists of the secondary circuit leads and the electrodes used to hold the crucible. Because the phase controlled chopper 103 only conducts during a portion of the mains input 101 alternating cycle, the phase controlled chopper 103 heavily loads the mains input voltage 101 by drawing large amounts of non-sinusoidal current. This often results in voltage disturbances to other devices connected to the same mains supply. Moreover, the non-sinusoidal current creates a poor power factor that increases the apparent power required to operate these devices.
A conventional EF utilizes 50 Hz-60 Hz power transformers and large copper conductive braided straps to create a mechanically flexible high current connection from the transformer to the electrodes. The flexible braids are required for allowing the electrodes to be separated for cleaning and inserting a new crucible for each analysis. The EF furnace uses a set of electrodes for delivering over 1100 Amps to a crucible. The magnetic loop created by the flexible leads connecting the transformer secondary to the electrodes produces substantial amounts of magnetic field that can couple into nearby objects. These magnetic fields can create interference with devices such as CRT monitors resulting in distortion of picture quality by altering the display position at the main frequency or one of its harmonics.
Often, the use of braid conductor at frequencies utilized by switching power supplies is not practical due to skin effect and large eddy currents resulting in extremely high temperatures in the connections. The high temperatures increase oxidation of the braid material further increasing its resistance. Moreover, the transformer's primary wires can also experience localized heating due to the large magnetic field created by the secondary current. In prior art devices, the high secondary current loop encircles only one side of the transformer creating magnetic fields that are not homogeneous over the entire structure. This often creates eddy current heating of the transformer's primary wires. The heated primary wire warms the transformer core. The added losses lower the amount of power the transformer can deliver before exceeding the transformer maximum operating temperature.
From a mechanical perspective, the size and weight of the 50 Hz-60 Hz transformer used in connection with thick copper braids result in increased package size and greater shipping cost. Although electronic solutions are known in the art for increasing the operating frequency to reduce transformer size, methods for reliably making such an electro-mechanical structure at the higher frequencies had not been realized. The problems involve realizing a flexible mechanical structure that minimizes inductance and loss of the high current secondary while providing reliable electrical contacts. The structure must allow repetitive insertion and removal of a crucible. Cleaning of the electrode assembly is also a requirement.
Alternative applications of using standard 50 Hz-60 Hz methods include using rigid bus bars and contacts to complete the electrical circuit. This includes using a conventional transformer high current secondary connected with conventional electrodes. This solution suffers from many of the problems outlined in previous paragraphs. Still further alternatives to the construction include the use of high current flexible conductors in the form of an S-bent conductive sheet. In this solution, the transformer is remote from the electrodes and the S-bent sheet is used to make the connections between the transformer secondary and the electrodes. A disadvantage to this type of arrangement includes excess inductance along with many problems as discussed previously.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.